Pipelines used for the transmission of oil and gas (and other fluids) need to be inspected. This may be immediately after construction or at periodic intervals thereafter. During the construction phase an obstruction in a pipeline may occur for any number of reasons. The pipeline may have received a blow externally from a machine for example, producing a dent extending into the pipe, a tool may have been left inside the pipe by a worker, excess sealant used at pipe joints may not have been properly removed, etc.
Pipelines are usually subjected to detailed gauge mapping before they are commissioned. However, a pig of this type is constructed with a metal body which may become stuck if it encounters an obstruction in the pipeline. To avoid the gauge pig becoming stuck, it is common practice to send a flexible foam pig through the pipeline first which can squeeze past obstructions much more easily. The foam pig is usually propelled through the pipeline by water. If the pig encounters an obstruction, the pig will in general be forced past the obstruction with and be damaged. If the foam pig exits the pipeline in a damaged condition, the operators know that it is not safe to deploy a (gauge) pig. Instead they would deploy further foam pigs to attempt to clean the line and remove obstructions until the foam pigs emerge from the line undamaged.
Pipelines are used to transport many different types of fluids including oil, gas, water, chemicals, slurries and food products. In order to operate efficiently, it is important that the bore of the pipeline does not become restricted either due to mechanical damage to the pipe wall or due to deposits building up on the inside of the pipeline. Even relatively small reductions in the bore, caused by deposits spread along the length of a pipeline, may have a significant effect on the flow of the product and the pumping efficiency. For this reason, cleaning pigs are used to remove deposits from the line. The pigs are in the form of a plug and they are pushed through the line by the product flow. The pig scrapes the deposits from the pipe wall and the particles are entrained by the flow of the product and pushed along by the pig to the end of the pipeline where they are removed by filters. In some pipelines, the cleaning process is carried out on a routine basis (say every week) and in others the operator will only use a cleaning pig when there is a noticeable fall in the pumping efficiency of the line. Typically, pigs are run through the line repeatedly until the product runs clean with no particles in the flow ahead of the pig. In all cases, the pipe is considered to be clean when this condition is reached.
Different types of pigs may be used during the cleaning process in order to avoid the possibility of a pig getting stuck. On the first run of a pig in the line, the degree of blockage of the line may be unknown and, therefore, early runs are carried out using very flexible foam pigs which are able to get through severe restrictions in the line. Once foam pigs are able to get through the line without damage or without too great a pumping pressure being required, then it is assumed that there are no major restrictions in the line and/or the quantity of deposits is not too great. At this point metal bodied pigs with plastic drive discs are used, which are more aggressive in removing deposits from the pipe wall. Finally, metal bodied pigs with brushes or metal scrapers may be used depending on the hardness and adhesion of the deposits.
In the oil and gas industry, the cleaning of pipelines is normally carried out ‘blind’ with no measurements being carried out to determine the nature and location of the blockage in the line or the thickness and the distribution of the deposits. As a result, the cleaning process must be carried out in a very cautious way, with multiple runs of foam pigs and a gradual progression to metal bodied pigs, with increasingly aggressive configurations. This approach can lead to significant inefficiencies in the cleaning process, with large numbers of cleaning runs being required and many of them possibly being unnecessary. It is proposed that a pig which can be used to measure the pipe bore, both at the start and during the cleaning process, could provide information to optimise the selection of the appropriate type of pig to be run in the line, at each stage in the process.
Instrumented pigs, which can measure and gauge the internal bore of a pipeline, are available and are commonly used in the oil and gas industry. These pigs have arrangements of sensors, transducers, and electronics to measure the internal bore of the pipeline and, by calibrating the systems, they are able to identify and measure changes in the pipe diameter. They also identify and measure features such as dents, ovality, and buckles, which may affect the integrity and operating performance of the pipeline. Typically, the product flow is used to push the pigs along and measurements are taken at frequent intervals, as the pigs are transported down the line. Measurement data are captured using recording devices on the pig and, at the end of the run, the data can be downloaded from the pig for viewing and analysis. However, all instrumented bore measurement pigs are hard bodied and are considered to be too great a risk for running in a pipeline at the early stages of cleaning. This patent proposes a bore measurement pig, which has a foam pig body, with the sensing system integrated into the foam matrix. This ensures that the device can be used with minimum risk, at any stage in the cleaning process, and will provide the operator with detailed information on the nature and location of blockages, and the thickness and distribution of deposits in the line.
It is known to instrument a foam pig. U.S. Pat. No. 5,659,142 describes a foam pig instrumented with pressure sensors. As the pig is caused to move along the pipeline a pressure log is recorded by a pressure sensor, which is housed in a cavity at the centre of the pig. The pressure sensor is part of a sealed unit also comprising a processor, a memory and a power source.
Whilst measuring pressure allows the position of an obstruction to be identified, relying on the measurement of pressure does not yield a great deal of information about the nature of the blockage. Also, some pipeline features, such as ovality, may not affect pressure. If the cross-sectional shape changes, without the cross-sectional area changing, there will be little effect on fluid pressure.
Ultrasonic sensing of pipeline geometry is known in the prior art. It has been shown to work very well as long as it is in a medium, e.g. water or other liquids, that have a predictable effect on the transmitted and reflected signal. In liquids that contain solids, gasses, or badly mixed phases, the transference of the signal through the medium can be unpredictable. This can cause confusion of the signal and severe measurement errors.
It would therefore be desirable to provide an instrumented foam pig that is capable not of not only measuring the position of obstructions, but also the position of changes in the shape of the pipeline, and also to yield more detailed information about the nature of obstructions and the size and shape of defects in the pipeline.